Sunday, 29 December 2019

RF Sniffer / level indicator

In the last post I talked about a low power 1W amplifier that I was building for WSPR TX. This amplifier has a variable gain - a simple potentiometer at the input, nothing fancy. This will set the output level.

The Amp output will go through a 40m LPF, on to my VSWR/Power meter, then my auto ATU and small loop antenna.

Wouldn't be nice to have some indication of the level of RF output? 

So I have looked into this and have come up with a row of LEDs, four green, one yellow and one red. The idea being that if the red one is alight the amp is close to overload.

To implement this I found on eBay an Audio Sound Level Indicator . I wired this to a simple peak detector and bingo it lights up when it detects RF.

The diodes are germanium low drop.

Here's a mock up, driven from a +12V supply, RF is detected on the short yellow wire at the bottom, when my ELAD FDM-DUO is transmitting at 7W power into my loop on the window sill!!!

Saturday, 7 December 2019

Nano VNA

There's a new kid on the block, and if you haven't noticed go buy one. It's the Nano VNA. A compact 50kHz to 900MHz network analyzer.

I have only just got to grips with this new toy, but already I can display my antenna SWR vs frequency. I get a best SWR of 1:2.8, so I must use my AutoATU, when I get < 1:1.5.


And here's an example using it to display the characteristics of 40m LPF I built from a kit.

The triangle marker is at the -3dB point, which shows top right as 8.3MHz for this filter.


To get this display I did this

DISPLAY - TRACE, turnoff all except Blue
SCALE - 10 x1